If it was made by Thomas Webb, I would say it is "Alexandrite" - the "tri-coloured, amber-rose-violet" I mentioned in the heat-sensitive topic that Sue linked to above. But I don't think the vase has the Webb quality I would usually expect.
It might be, confusingly (as pointed out by Glen and Terry in reference to the Revi book), an item produced under Locke's "amberina" patent that provided for the "... development of a violet shade ...", but not as a "plated ware" as Revi sates in the part quoted by Glen.
The vase does not seem to be "plated", in other words, having an applied separate colour. Therefore the blue (violet) at the rim is the result of that part receiving a further reheating after the initial reheating had produced the "amberina" colouring.
Or, I suppose it could have been intended a basic "amberina" piece but that the rim received just enough extra heat to cause it to strike the "rose" into "violet". A bit like the sort of pleasant "accident" that first resulted in two and three coloured pieces through reheating? Could that possible, or going back to sqaure one, would it have been a planned result having mixed the batch to produce exactly that colouring?